I don't know if there's an ideal spot, but based on what I've read, I think an optimal lineup would be:
1. Xander
2. JBJ
3. Mookie
4. Ortiz
5. Pedroia
6. Hanley
7. Leon (He is a wildcard right now based on his outrageous numbers in a small sample.)
8. Shaw
9. Holt
That said, I am not one for changing the line up around because it just doesn't make that big of a difference. Also, our top 6 are all pretty good, and could be interchanged without much damage, comfort level aside.